The present invention relates to a vehicle steering system which can perform an automatic steering control and cancel the control.
There are proposed techniques for performing an automatic steering of a vehicle using a steering actuator provided on the vehicle. For example, a power steering system installed on a vehicle includes an electric motor or a hydraulic actuator as a steering actuator for imparting a steering force (a steering assist force) to a steering mechanism. Then, steered road wheels of the vehicle can be turned by controlling the output of the steering actuator without involving the manipulation of a steering wheel by the driver.
More specifically, in an automatic parking assist control, a target traveling path from a current position to a parking position of the vehicle is calculated, and a relationship of a target turning angle with a traveling distance of the vehicle is obtained so that the vehicle is reversed along the target traveling path so calculated. Then, the steering actuator is controlled based on a steering angle command value which is determined so as to attain a target turning angle according to a traveling distance of the vehicle. At this time, the driver only has to perform the adjustment of stopping position as well as the adjustment of vehicle speed by controlling the brake and accelerator pedals and does not have to manipulate the steering wheel.
When the steered road wheel strikes an obstacle such as a curbstone during the automatic parking control, since the steering actuator cannot steer the steering mechanism, an actual steering angle cannot follow a steering angle command value. In addition to this, in the case of an electric motor being used as the steering actuator, such a state occurs in which a maximum current continues to be supplied to the electric motor, the electric motor being thereby put in an overheat state. Then, in a related art technique disclosed in JP-A-2004-249913, in response to the fact that a steering angle detection value does not reach the steering angle command value and an accumulated time period during which a motor current value is large has reached a predetermined determination time period, it is determined that a steering disablement state occurs and the automatic parking control is cancelled in the midst thereof.
In the case of the automatic parking control mentioned above, the changing speed of the target steering angle of the steering mechanism may become fast depending upon the adjustment of vehicle speed by the driver, the approaching angle to the parking position or the target traveling path. Consequently, although the steering actuator has to drive the steering mechanism at fast speeds, due to output shortage of the steering actuator, the actual steering angle cannot be caused to follow the target steering angle, and this may lead to a state in which a deviation therebetween remains large. In the related art technique mentioned above, even though such a state occurs, the automatic parking control is cancelled.
Thus, the conventional automatic steering function such as automatic parking has a tendency that the control is easily cancelled, and therefore, this function is not necessarily easy for the driver to use.